1. Field of the Invention
This invention relates to a method of an apparatus for distinguishing the types of pixels making up an original, and more particularly to a method of and an apparatus for distinguishing the types of pixels making up an original, which are used, for instance, in a stencil printer which makes a stencil by imagewise perforating a heat-sensitive stencil material and a print is made from the stencil, and a copier or a printer in which a latent image is formed on a photosensitive medium by electrophotography or the like and a toner image obtained by developing the latent image is transferred to a printing paper or a heat-sensitive paper.
2. Description of the Related Art
In the fields of plate making and printing, there has been put into practice a process in which an original, in which binary images such as a line drawing and characters and tone images such as a picture and halftone dots mingle together (such an original will be referred to as a “complex original”, hereinbelow), is read by an image scanner, a multivalued image signal sampled in a main scanning direction and a sub-scanning direction pixel by pixel is obtained, the multivalued image signal is converted into a binary image signal, and a plate or a print is made from the binary image signal.
In order to obtain a desirable output when a complex original is output through an image processing apparatus, generally, the region of the binary image is subjected to a binary image density conversion in which the density of each pixel is converted to a maximum density or a minimum density on the basis of a single threshold value, the region of the tone image is subjected to a tone image density conversion in which the density of each pixel is converted taking into account the properties of the input/output system so that the tone properties of the original image are preserved, and then the region of the binary image is binary-coded by a simple binary-coding method using a single threshold value while the region of the tone image is binary-coded by a pseudo-halftone expressing method such as a dither method and an error diffusion method. Further, since moiré is apt to be generated when a halftone picture is binary-coded by the dither method, it is not preferred that the picture region and the halftone region are subjected to density conversion by the use of the same characteristics and are binary-coded by the same method.
Accordingly, it is necessary to determine the type of each region, that is, to distinguish the binary image region such as a character region from the tone image region such as a halftone region and a picture region. In other words, it is necessary to distinguish the type of each pixel (the type of image which the pixel makes up) and to carry out on each pixel image processing suitable to the type of pixel. For example, pixels of a binary image should be subjected to a density conversion for a binary image so that characters are output to be high indensity and pixels of a tone image should be subjected to a density conversion for a picture or a halftone image so that the tone properties of the original image are preserved.
When a complex original is used, it is necessary to precisely distinguish the types of pixels, that is, to distinguish pixels of a character, pixels of a picture or pixels of a halftone image from each other, for an image signal of one frame corresponding to one sheet of original, and to carry out image processing optimal to each type of pixels. There have been proposed various methods of distinguishing the character region, the picture region and the halftone region from each other.
For example, as a method of distinguishing whether a pixel is of a half tone region, there have been proposed various methods as follows. In the method disclosed , for instance, in Japanese Unexamined Patent Publication Nos. 2(1990)-274174 and 5(1993)-344331, whether a pixel is of a halftone region is determined on the basis of the number of edges in a reference region of a predetermined size. In the method disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 60(1985)-51367, 62(1987)-88478, 5(1993)-48891 and 6(1994)-152944, whether a pixel is of a halftone region is determined on the basis of information on the space between edges or extremes (a maximum or a minimum) of density. In the method disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 3(1991)-80770 and 5(1993)-110831, whether a pixel is of a halftone region is determined pattern matching.
As an apparatus for carrying out the method in which whether a pixel is of a halftone region is determined on the basis of the number of edges in a reference region of a predetermined size, there has been known an apparatus shown in FIG. 11. As shown in FIG. 11, the apparatus comprises an edge detecting means 41, a main scanning direction halftone pixel determining means 44 and a sub-scanning direction half tone pixel determining means 45.
In the apparatus, whether a pixel is of a halftone region is determined according to an algorithm that the halftone region differs from the character region and the silver halide picture region in number and arrangement of edges.
Specifically, the edge detecting means 41 detects edges on the basis of the values of a designated one of pixels and a plurality of pixels near the designated pixel (will be sometimes referred to as “the neighbor pixels”, hereinbelow) with substantially all the pixels designated in sequence. The main scanning direction halftone pixel determining means 44 determines, on the basis of the result of edge detection by the edge detecting means 41, a region where at least a predetermined number of edges are arranged on one main scanning line spaced from each other within a predetermined distance to be a prospective halftone region, and takes the pixels in the prospective halftone region as prospective halftone pixels. Since a larger number of edges exist in the halftone region than in the character region and the picture region, the former can be distinguished from the latter by this processing. Then the sub-scanning direction halftone pixel determining means 45 counts a number of the prospective halftone pixels in a reference region which includes one pixel in the main scanning direction and a predetermined number of pixels in the sub-scanning direction with a designated pixel at the middle thereof, and determines the designated pixel to be a halftone pixel when the number of the prospective halftone is not smaller than a predetermined threshold value. Even pixels which are determined to be a prospective halftone pixels by the main scanning direction halftone pixel determining means 44 are not determined to be a halftone pixel unless the pixel satisfies the condition in the sub-scanning direction. That is, according to this algorithm, whether each pixel is a halftone pixel is determined on the basis of the result of edge detection.
Some halftone pictures have a relatively high density portion which is expressed by a number of ink dots (such a portion will be referred to as “a solid portion”, hereinbelow) and smoothly merges into a halftone region expressed by halftone dots as shown in FIGS. 3A and 4A. When distinguishment of the type of pixel is carried out on a complex original including a halftone picture having such a solid portion according to the conventional algorithm, the solid portion is not determined to be a halftone region (a tone image region) but determined to be a non-halftone region, e.g., a character region (a binary image region), since there is detected no edge component in the solid portion as shown in FIG. 3B.
When the image signal of the original is converted into a binary image signal on the basis of the distinguishment, the solid portion is subjected to the binary image density conversion and is output to be high in density while the halftone region is output to be lower in density than the solid portion. As a result, a sharp difference in density is generated between the solid portion and the halftone region, which gives an observer a sense of incompatibility.